Flexible disks, known as floppy disks, are generally made of plastic and are utilized for recording and reading information usually in an off-line mode in a data processing system. These flexible disks are light weight, relatively low cost and convenient to use. The disks are packaged in envelopes formed from sheets of flexible plastic such as polyvinyl chloride. The envelopes have aligned center or drive holes which allow the enclosed flexible disk to be rotated within the envelope. The envelope is lined with layers of a low friction liner material. The envelope includes slots which permit writing and reading into and from the disk.
The flexible plastic envelopes are formed from precut and prepunched sheets of plastic material which has the center drive holes and access slots prepunched therein. Half of a precut sheet or blank is rectangular and the other half is identical, except that it includes three flaps which extend from its three outer edges. The low friction liner or wiping material is attached to the inner surface of the sheet prior to cutting and punching. The envelope is formed by making a centerfold therein by folding each half of the sheet against the other half. The side flaps are folded around the edges of the opposite half of the sheet and are sealed thereto. A flexible disk is then inserted into the envelope and the third or end flap is then folded to complete assembly of the flexible disk and envelope. The centerfold and the side flap folds must be exactly located and formed in order to meet the specifications for the flexible disk unit. If the folds are of too small a radius, the friction between the disk and the liner will increase the torque required by the flexible disk drive system. Too large a fold radius will result in insufficient friction of wiping material against the disk resulting in an out-of-specification flexible disk unit. Warpage of the completed flexible disk unit must be kept within tight tolerances to produce an acceptable flexible disk system.
The forming of the side folds around the mandrel and then heating the fold to cause annealing of the plastic material and then bonding or sealing the flap to the underlying plastic surface has resulted in a large number of rejections of the final diskettes because of the tendency of the material that is folded to have a memory which causes it to unbend slightly thereby causing the underlying surface to which it is bonded to also rise thereby giving a bulge in the envelope.
Accordingly, it is the main feature of the present invention to overstress the side flap fold bend so that the memory will bring it back to the plane which is desired and eliminate the undesired bulge.
It is a further feature of the present invention to provide a radiused edge portion on the mandrel about which the fold is made.
It is another feature of the present invention to provide a folding apparatus which forms the fold and also provides overstressing of the material at the fold to compensate for material memory return.